1. Field of the Invention
The present invention relates to a switching power supply device in a power IC to which an activation terminal and a drain terminal of a power MOSFET are commonly connected. In order to avoid a phenomenon that a time required for releasing an operation prohibited state of a switching element after operating an overvoltage protection circuit and then turning off an alternating current input voltage is extremely long, the switching power supply device of the present invention monitors the alternating current input voltage, and releases the operation prohibited state of the switching element when the alternating current input voltage drops to a threshold value or less, and thereby releases the operation prohibited state of the switching element instantaneously after turning off an alternating current power supply even in a high-power power supply system in which a capacitance value of an input smoothing capacitor is large.
2. Description of the Related Art
As a conventional switching power supply device, there is a device that protects a switching element in the following manner when a secondary side output voltage becomes an overvoltage owing to such an abnormal state that an output voltage detection circuit turns to an open state. Specifically, for the purpose of protecting the switching element, such an overvoltage state of the secondary side output voltage is indirectly detected by a primary side power supply voltage through a transformer, a switching operation is then stopped, and an operation prohibited state is maintained for a while until an AC plug is pulled out.
FIG. 1 is a circuit diagram showing an example of this type of switching power supply device. This switching power supply device includes an input rectifying/smoothing circuit 10, a transformer 20, a primary winding 21 of the transformer 20, a drive winding 23, an activation circuit 30, a first switch 31, a Zener diode 32, an inverter 33, a second switch 34, a rectifying/smoothing circuit 40, a low-voltage malfunction prevention circuit 50, a reference voltage circuit 60, a feedback control circuit 70, a switching element 80, an overcurrent detection circuit 90, a PWM control circuit 100, a NOR gate 110, a drive circuit 120, a low-input protection circuit 160, an overvoltage protection circuit 180, and an integrated circuit 190e composed of a control circuit.
Next, a description is made of configurations and operations of the respective units by using a timing chart shown in FIG. 2. First, an alternating current input voltage V1 of an alternating current power supply is inputted to the input rectifying/smoothing circuit 10. Then, the alternating current input voltage V1 is rectified by a diode bridge composed of diodes 11 to 14, is thereafter smoothed by a smoothing capacitor 15, and is outputted as a direct current input voltage V2 from the input rectifying/smoothing circuit 10. Then, the direct current input voltage V2 is supplied to the switching element 80 through the primary winding 21 of the transformer 20.
When the switching element 80 is switched on and off, the direct current input voltage V2 is converted into an alternating current voltage. Then, energy is transmitted from the primary winding 21 of the transformer 20 to a secondary winding 22 of the transformer 20 and the drive winding 23.
When the direct current input voltage V2 rises and reaches a breakdown voltage V3 of the Zener diode 32, the activation circuit 30 supplies an activation current I3 to a capacitor 42 of the rectifying/smoothing circuit 40.
Then, at the time of activation, the activation current I3 charges the capacitor 42 of the rectifying/smoothing circuit 40. In such a way, a power supply voltage V4 is raised. Moreover, after the switching operation is started, the power supply voltage V4 is generated from a voltage induced by the drive winding 23 of the rectifying/smoothing circuit 40, and is supplied to the integrated circuit 190e composed of the control circuit.
The power supply voltage V4 reaches a first threshold value V5a of the low-voltage malfunction prevention circuit 50. Then, the reference voltage circuit (REG) 60 operates, and a reference voltage V6 is supplied to the respective circuit blocks. In such a way, the switching element 80 starts the switching operation. At this time, the second switch 34 is simultaneously turned to the open state through the inverter 33, whereby the activation current I3 is stopped.
A current detection resistor 91 of the overcurrent detection circuit 90 detects, as a voltage, a current I1 with a triangular waveform, which flows during an ON period of the switching element 80. Then, a low pass filter 92 of the overcurrent detection circuit 90 only passes a low frequency component of the detected voltage therethrough, and outputs an overcurrent signal V8 to a comparator 75.
The feedback control circuit 70 generates a control voltage V7 based on an error amplified signal from an output voltage detection circuit 140. Here, the output voltage detection circuit 140 includes photocouplers 141a and 141b, a resistor 142, and a Zener diode 143. Then, the PWM comparator 75 of the feedback control circuit 70 compares the control voltage V7 and the overcurrent signal V8 with each other. Then, when the overcurrent signal V8 rises to the control voltage V7 or more, an OFF trigger signal of the switching element 80 is outputted to the PWM control circuit 100. In such a way, a PWM control of the switching element 80 is performed.
The low-input protection circuit 160 is a circuit for preventing excessive spread of an ON duty of the switching element 80 and a breakage of the switching element 80 when a low voltage is applied as the alternating current input voltage V1 owing to adverse power supply circumstances and the like. The low-input protection circuit 160 is composed of two voltage dividing resistors 161 and 162, a smoothing capacitor 163, and an input detection comparator 164.
The alternating current input voltage V1 is subjected to half-wave rectification by the diodes 12 and 14, is thereafter dropped by the two voltage dividing resistors 161 and 162, and is supplied to the smoothing capacitor 163. Then, the smoothing capacitor 163 smoothes the dropped half-wave rectified voltage, and generates an input detection signal V10 proportional to the alternating current input voltage V1. In such a way, the input detection signal V10 is inputted to the input detection comparator 164. The input detection comparator 164 compares the input detection signal V10 and an input detection threshold value V9 with each other. Then, when the input detection signal V10 falls down below the input detection threshold value V9, the input detection comparator 164 determines that the present state is a low input state, and switches an operation prohibition signal V12 from an L level to an H level. In such a way, the switching operation of the switching element 80 is stopped, and the switching element 80 is protected.
When such abnormality that the output voltage detection circuit 140 turns to the open state occurs, an output voltage V11 sometimes rises to the threshold value or more. At this time, the overvoltage protection circuit 180 indirectly detects an overvoltage state of the output voltage V11 through the transformer 20 by the power supply voltage V4. In such a way, the overvoltage protection circuit 180 stops the switching operation of the switching element 80, and performs overvoltage protection. Next, a description is made of detailed operations of the overvoltage protection circuit while referring to the timing chart of FIG. 2.
First, at the time of a stationary operation, a voltage, as the power supply voltage V4, of which magnitude is a product of a turns ratio of the drive winding 23 and the secondary winding 22 and the output voltage V11 decided by a breakdown voltage of the Zener diode 143 provided in the output voltage detection circuit 140 is generated.
Here, when such abnormality that the output voltage detection circuit 140 turns to the open state occurs, the output voltage V11 rises (overvoltage state). Then, the power supply voltage V4 also rises in proportion to the output voltage V11. Then, when the power supply voltage V4 rises to an overvoltage detection threshold value V13 or more, an overvoltage detection comparator 181 outputs a SET signal to a SET terminal of an overvoltage latch 182. Then, an overvoltage operation prohibition signal V14 switches from the L level to the H level.
At this time, the overvoltage operation prohibition signal V14 prohibits the switching operation of the switching element 80 through a NOR gate 110, and at the same time, turns the first switch 31 to a conductive state.
Thereafter, when the power supply voltage V4 drops to the threshold voltage decided in the activation circuit 30, the activation circuit 30 supplies the activation current I3. In such a way, the power supply voltage V4 is prevented from dropping to a release threshold value V18 of the overvoltage latch 182. In such a way, the operation prohibited state of the switching element 80 is maintained (operation prohibition period).
Next, the alternating current input voltage V1 is turned OFF in order to release the operation prohibited state. Then, the direct current input voltage V2 drops slowly while discharging the smoothing capacitor 15. Then, when the direct current input voltage V2 drops to the breakdown voltage V3 of the Zener diode 32, the activation current I3 is stopped. Therefore, the power supply voltage V4 starts to drop (alternating current power supply OFF period).
When the power supply voltage V4 drops to the overvoltage latch release threshold value V18 or less, a latch release comparator 183 outputs a release signal to a RESET terminal of the overvoltage latch 182. In such a way, the overvoltage operation prohibition signal V14 switches from the H level to the L level, and the operation prohibited state is released (operation prohibition release period).
Incidentally, it is considered ideal that the operation prohibition release period as a time for releasing the operation prohibited state after the overvoltage is detected and the alternating current input voltage is then turned OFF be as short as possible.
Moreover, as a related technology of the conventional switching power supply device, an uninterruptible power supply device described in Patent Publication 1 is known. This uninterruptible power supply device stops the power supply operation once after an output overvoltage is detected, then detects a pulsating voltage of an alternating current input, and automatically resumes the power supply for each cycle of the alternating current power supply. Then, in the case where the overvoltage state continues, such automatic resuming is stopped after counting the number of automatic resuming times.    [Patent Publication 1] Japanese Patent Laid-Open Publication No. 2007-268501